ANALYSIS AND DESIGN

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CIS460


NETWORK
ANALYSIS AND DESIGN

CHAPTER 7
-


Selecting Bridging, Switching, and
Routing Protocols

Introduction


In this chapter we are going to look at bridging,
switching, and routing protocol attributes of:


Network Traffic characteristics


Bandwidth, memory, and CPU usage


The approximate number of peer routers or switches
supported


The capability to quickly adapt to changes in an
internetwork


The capability to authenticate route updates for security
reasons

Making Decisions as Part of the Top
-
Down Network Design Process


Factors involved in making sound
decisions:


Goals must be established


Many options should be explored


The consequences of the decisions should be
investigated


Contingency plans should be made


Use a decision to match options with goals

Making Decisions as Part of the Top
-
Down Network Design Process (Cont’d)


Table 7
-
1 shows a decision table


Once decision is made look at it to determine:


What could go wrong


Hs it been tried before


How will customer react


Contingency plans if customer disapproves


Can use during both logical and physical design
phase

Selecting Bridging and Switching
Methods


Decision making is simple because of few options


If includes Ethernet bridges and switches most likely use
transparent bridging with spanning
-
tree protocol


Might also need a protocol for connecting switches that
support virtual LANs


With Token Ring networks options include source
-
route
bridging (SRB), source
-
route transparent (SRT) bridging
and source
-
route switching (SRS)

Characterizing Bridging and
Switching Methods


Bridges operate at Layers 1 and 2 of OSI


Determine how to forward a frame based on
information in Layer 2 header


Bridge does not look at Layer 3 information


Bridge segments bandwidth domains so that devices
do not compete with each other for media access
control


Bridge does forward Ethernet collisions or MAC
frames in a Token Ring network


Characterizing Bridging and
Switching Methods (Cont’d)


Bridge does not segment broadcast domains. It sends
broadcast packets out all ports


Bridges normally connect like networks but can be a
translation or encapsulating bridge


A switch is like a bridge only faster


Switches take advantage of fast integrated circuits to
offer very low latency


Switches usually have a higher port density and a
lower cost per port

Characterizing Bridging and
Switching Methods (Cont’d)


Bridges do store and forward


Switches can be store and forward or cut
-
through


Cut
-
through is faster but more prone to
letting runts or error packets through


On a network that is prone to errors do not
use cut
-
through processing


Adaptive cut
-
through switching

Transparent Bridging


Most common Ethernet environments


A transparent bridge (switch) connects one
or more LAN segments so that end systems
on different segments can communicate
with each other transparently


Looks at the source address in each frame to
learn location of network devices


It develops a switching table (Table 7
-
2)

Transparent Bridging (Cont’d)


Receives a packet look sup address in switch table


If no address it sends the frame out every port like
a broadcast frame


Send Bridge Protocol Data Unit (BPDU) frames to
each other to build and maintain the spanning tree


Sends BPDU to a multicast address every two
seconds


Source
-
Route Bridging


Developed for Token Ring networks in the 80s by
IBM


Uses a source
-
routing
-
transparent (SRT) standard


An SRT bridge can act like a transparent bridge or
a source
-
routing bridge depending on whether
source
-
routing information is included in a frame


Not transparent if pure SRB is used

Source
-
Route Bridging (Cont’d)


Uses explorer frames


All
-
routes explorer
-

take all possible paths, take just
one route back


Single
-
route explorer
-

takes just one path and
response take all paths or just one back


With single
-
route explorer frames the spanning
-
tree
algorithm can be used to determine a single path


Scalability is impacted by amount of traffic when
all
-
routes explorer frames are used

Source
-
Route Switching


SRS is based on SRT bridging


SRS forwards a frame that has no routing
information field


Learns the MAC addresses of devices on the ring


Also learns source
-
routing information for devices
on the other side of SRB bridges

Source
-
Route Switching (Cont’d)


Benefits


Rings can be segmented without adding new ring
numbers


can be incrementally upgraded to transparent bridging
with minimal disruption or reconfiguration


does not need to learn the MAC addresses of devices on
the other side of source
-
route bridges


can support parallel source routing paths


can support duplicate MAC addresses

Mixed
-
Media Bridging


Mixture of Token Ring, FDDI and Ethernet bridging


Encapsulating bridging is simpler than translation
bridging but is only appropriate for some network
topologies


Encapsulating bridge encapsulates an Ethernet frame
inside an FDDI or Token ring frame for transversal
across a backbone network that has no end systems

Mixed
-
Media Bridging (Cont’d)


Support for end systems on a backbone then
need to use translation bridging which translates
from one data
-
link
-
layer protocol to another


Problems


Incompatible bit ordering


Embedded MAC addresses


Incompatible maximum transfer unit (MTU) sizes


Handling of exclusive Token Ring and FDDI functions


No real standardization

Mixed
-
Media Bridging (Cont’d)


While FDDI is a common choice for
backbone networks in campus network
designs to avoid translating Ethernet and
FDDI frames should use 100
-
Mbps Ethernet
or Gigabit Ethernet on backbone segments

Switching Protocols for
Transporting VLAN Information


When VLANs are implemented in a switched network
the switches need a method to make sure intra
-
VLAN
traffic goes to the correct segments


Accomplished by tagging frames with VLAN
information


two tagging methods:


adaptation of the IEEE 802.10 security protocol


Inter
-
Switch Link (ISL) protocol

IEEE 802.10


A security specification used as a way of placing
VLAN identification (VLAN ID) in a frame


Inserted between the MAC and LLC headers of
the frame


The VLAN ID allows switches and routers to
selectively forward packets to ports with the same
VLAN ID


VLAN ID removed from frame when forwarded
to destination segment

Inter
-
Switch Protocol


Another method for maintaining VLAN
information as traffic goes between switches


Developed to carry VLAN information on a 100
-
Mbps Ethernet switch
-
to
-
switch or switch
-
to
-
router link. Can carry multiple VLANs


ISL link is call a trunk. A trunk is a physical link
that carries the traffic of multiple VLANs between
two switches or between a switch and a router.
Allows VLANs to extend across switches

VLAN Trunk Protocol


Some networks have a combination of different
media types


VLAN trunk protocol (VTP) allows a VLAN to
span the different technologies by automatically
configuring a VLAN across a campus network
regardless of media type


VTP is a switch
-
to
-
switch and switch
-
to
-
router
VLAN management protocol that exchanges
VLAN configuration changes as they are made to
the network

Selecting Routing Protocols


A routing protocol lets a router dynamically
learn how to reach other networks and
exchange this information with other routers or
hosts


Selecting routing protocols is harder than
selecting bridging protocols because there are
so many


Made easier using a table such as 7
-
1 to pick
the best one


Characterizing Routing Protocols


General goal to share network reachability
information among routers


Some send complete other only an update


Differ in scalability and performance
characteristics


Many are designed for small networks


Static environment


Some are meant for connecting interior campus
networks

Distance
-
Vector Versus Link
-
State Routing Protocols


Two major classes: distance
-
vector and link
-
state


Distance
-
vector protocols


IP Routing Information Protocol (RIP) Version 1 and 2


IP Interior Gateway Routing Protocol (IGRP)


Novell NetWare Internetwork Packet Exchange Routing
Information Protocol (IPX RIP)


AppleTalk Routing Table Maintenance Protocol (RTMP)


AppleTalk Update
-
Based Routing Protocol (AURP)


IP Enhanced IGRP


IP Border Gateway Protocol (BGP) (path
-
vector)

Distance
-
Vector Versus Link
-
State Routing Protocols (Cont’d)


Vector means distance or course. A distance
-
vector includes information on the length of the
course. Many use hop count


A hop count specifies the number of routers that
must be traversed


Maintains a distance
-
vector routing table that
lists know networks and the distance to each.


Sends table to all neighbors, or an update after
first transmission


Distance
-
Vector (Cont’d)


Split Horizon, Hold
-
Down, and Poison
-
Reverse
Features


Split
-
horizon technique
-

sends only routes that are
reachable via other ports


Hold
-
down timer
-

new information about a route to a
suspect network is not believed right away. A standard
way to avoid loops


Poison
-
reverse messages
-

way of speeding convergence
and avoiding loops. When a router notices a problem it
can immediately send a route update that specifies the
destination is no longer reachable

Link
-
State Routing Protocols


Do not exchange routing tables


Exchange information about the status of their
directly connected links using periodic multicast
messages


Each router builds its own routing table


Protocols


IP Open Shortest Path First (OSFP)


IP Intermediate System
-
to
-
Intermediate System (IS
-
IS)


NetWare Link Services Protocol (NLSP)


Link
-
State Routing Protocols
(Cont’d)


Converge more quickly


Less prone to routing loops


Require more CPU power and memory


More expensive to implement and support


Harder to troubleshoot


Routing Protocol Metrics


Used to determine which path is preferable
when more than one path is available


Vary on which metrics are supported


Distance
-
vector use hop count


Newer protocols take into account delay,
bandwidth, reliability and other factors


Metrics can effect scalability

Hierarchical Versus Non
-
Hierarchical Routing Protocols


Some routing protocols do not support hierarchy


Normally all routers perform same tasks


Hierarchical protocols assign different tasks to
different routers and group routers in areas


Some routers communicate with local routers in
the same area and other routers have the hob of
connecting areas, domains, or autonomous
systems

Interior Versus Exterior Routing
Protocols


Interior protocols, such as RIP, OSPF, and
IGRP are used by routers within the same
enterprise or autonomous


Exterior such as BGP perform routing
between multiple autonomous systems.

Classful Versus Classless
Routing Protocols


A classful routing protocol always considers
the IP network class


Address summarization is automatic by major
network number and discontiguous subnets are
not visible to each other


Classless protocols transmit prefix
-
length or
subnet mask information with IP network
addresses. The IP address can be mapped so
that discontinuous subnets and VLSM are
supported

Dynamic Versus Static and
Default Routing


Static routes are often used to connect to a
stub network


A stub network is a part of an internetwork
that can only be reached by one path


Internal routers can simply be configured
with a default route that points to the ISP

Scalability Constraints for
Routing Protocols


Consider customer’s goals for scaling the
network to a larger size


There are a number of questions that relate
to scalability that should be answered


They can be answered by watching routing
protocol behavior with a protocol analyzer
and by studying the relevant specifications

Routing Protocols Convergence


Convergence is the time it takes for routers
to arrive at a consistent understanding of the
internetwork topology after a change takes
place


Understand the frequency of changes, links
that fail often, etc


Convergence time is a critical design
constraint

Routing Protocols Convergence
(Cont’d)


Convergence starts when a router notices a link
has failed


If a serial link fails it can start immediately. If
it uses keepalive frames it starts convergence
after it has been unable to send two or three
keepalive frames


If use hello packets and the hello timer is
shorter than the keep alive timer then routing
protocol it can start convergence sooner

IP Routing


Most common protocols are RIP, IGRP,
Enhanced IGRP, OSPF, and BGP

Routing Information Protocol


The first standard routing protocol developed for
TCP/IP environments


It is a distance
-
vector protocol that features
simplicity and ease
-
of
-
troubleshooting


Uses a hop count to measure the distance to a
destination. Cannot be more than 15 hops


RIPv2 developed to address some of the
scalability and performance problems with
Version 1


Interior Gateway Routing
Protocol


Meet needs of customers requiring a robust
and scalable interior routing protocol


Uses composite metric based on:
bandwidth, delay, reliability, and load


Load balances over equal
-
metric paths and
non
-
equal
-
metric paths. (3 to 1)


Has a better algorithm for advertising and
selecting a default rout than RIP

Enhanced Interior Gateway
Routing Protocol


Meet the needs of enterprise customers with
large, complex, multiprotocol internetworks


Goal is to offer quick convergence on large
networks. Diffusing update algorithm (DUAL)
guarantees a loop
-
free topology


The router develops a topology table that
contains all destinations advertised by
neighboring routers. It can scale to thousands
of nodes


Open Shortest Path First


Open standard supported by many vendors


converges quickly


authenticates protocol exchanges to meet
security goals


supports discontiguous subnets and VLSM


sends multicast frames vice broadcast frames


does not use a log of bandwidth


can be designed in hierarchical areas

Open Shortest Path First (Cont’d)


Propagates only changes


accumulate link
-
state information to calculate
the shortest path to a destination


all routers run the same algorithm in parallel


Allows sets of networks to be grouped into
areas


A contiguous backbone area, called Area ) is
required


Assign network numbers in blocks that can be
summarized

Border Gate Protocol


iBGP used at large companies to route
between domains


EBGP is often used to multihome an
enterprise’s connection to the Internet


Main goal is to allow routers to exchange
information on paths to destination
networks

Apple Talk Routing


Three options:


Routing Table Maintenance Protocol (RTMP)


AppleTalk Update
-
Based Routing Protocol
(AURP)


Enhanced IGRP for AppleTalk


RTMP is most common because it is easiest to
configure and is supported by most vendors

Routing Table Maintenance
Protocol


Routing table sent every 10 seconds using
split horizon


Works closely with Zone Information
Protocol (ZIP)


Checks routing table updates and sends ZIP
query

Using Multiple Routing and
Bridging Protocols


Important to realize you do not have to use
the same routing and bridging protocols
throughout the internetwork


To merge old networks with new networks
it is often necessary to run more than one
routing or bridging protocol


Solutions include source
-
route transparent
bridging, external routes in OSPF and RIP2


Redistribution between Routing
Protocols


Redistribution allows a router to run more than
one routing protocol and share routes among
routing protocols


Network administrator must configure
redistribution by specifying which protocols
should insert routing information into other
protocol’s routing tables


A router can learn about a destination from
more than one protocol

Integrated Routing and Bridging


CISCO offers support for IRB which
connects VLANs and bridged networks to
routed networks within the same router


One advantage of IRD is that a bridged IP
subnet or VLAN can span a router

Summary


Deciding on the right bridging, switching,
and routing protocols for your customer will
help you select the best switch and router
products for the customer